Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) typically include a "fail-safe" mechanical braking system that is applied in the absence of electrical power in the utility grid, or excessively fast rotation of the turbine hub (overspeed). However, for wind turbines having large rotors (e.g., 30 or more meters in diameter), application of a mechanical brake results in structural loads to the turbine blades, the gear box and the tower. These structural loads shorten the useful life of the wind turbine.
One braking solution is aerodynamic braking of the turbine by pitching the blades of the turbine to reduce rotational speed of the hub. Aerodynamic braking reduces structural loads applied to components of the wind turbine when power is provided by the utility grid. However, when power is not supplied by the utility grid, the mechanical fail-safe braking system is applied to stop rotation of the wind turbine. In an environment where utility grid interruptions are common, caused by, for example, multiple power outages, poor line conditions, or other interruptions, frequent use of the mechanical braking system can shorten the useful life of a wind turbine.
What is needed is a braking system for a wind turbine that applies a mechanical brake for overspeed conditions during short power outages and not automatically upon loss of power.